1. Field of the Invention
The present invention relates to a high frequency apparatus and a method for manufacturing the same high frequency apparatus. In particular, the present invention relates to a high frequency apparatus for transmitting or processing a high frequency signal of microwave, sub-millimeter wave, millimeter wave or the like, such as a high frequency transmission line, a high frequency device, a high frequency circuit or the like, and to a method for manufacturing the same high frequency apparatus.
2. Description of the Related Art
In recent years, with increasing desire for improvement in high frequency transmission techniques, as a prior art relating to high frequency transmission lines for microwaves, sub-millimeter waves and millimeter waves, there has been proposed a microstrip millimeter waveguide (hereinafter, referred to as a first prior art), disclosed in FIG. 1 of the Japanese Patent Laid-Open Publication No, JP-10-163711-A.
The microstrip type millimeter waveguide according to this first prior art is characterized by having the following structure:
(a) a first single crystal substrate has a recess formed therein by an anisotropic etching;
(b) a conductor is stacked as a grounding surface on a surface on which the recess is formed;
(c) a second single crystal substrate has a first microstrip line conductor and a conductor serving as a grounding surface, where the first microstrip conductor is formed on one surface of the second single crystal substrate, and the conductor is formed on another surface thereof which connects with the first single crystal substrate; and
(d) the first and second single crystal substrates are connected with each other in such a way that a first microstrip line provided on the second single crystal substrate is placed on the recess formed in the first single crystal substrate.
That is, this microstrip type millimeter waveguide has such a constitution that a strip conductor of the microstrip line formed on the second single crystal substrate and the grounding conductor film formed on the recess of the first single crystal substrate are formed via an air space between the second single crystal substrate and the grounding conductor film.
Also, in a high frequency passive circuit for processing a high frequency signal of microwave, sub-millimeter wave, millimeter wave or the like according to a prior art, in order to reduce the insertion loss, either a semiconductor substrate such as a gallium arsenide substrate or a dielectric substrate having a low dielectric constant such as a sapphire substrate is used, and moreover, the thickness of the substrate is made to be thin. However, the dielectric substrate having the low dielectric constant is generally high priced, and thinning of the dielectric substrate can be done up to at most about 100 μm, and there is such a limitation on the improvement in the electrical performance in the high frequency bands. On the other hand, a semiconductor substrate such as a low-priced semiconductor substrate has a large dielectric loss such that there can not be obtained any enough electrical characteristic.
In recent years, attention has been paid to so-called RF MEMS (Radio Frequency Micro-Electro-Mechanical-Systems) devices, which are high frequency devices using the micromachining technique. Since this technique is capable of fabricating a high aspect structure and a membrane structure, even if a high frequency circuit is fabricated on a low-priced silicon substrate, the high frequency circuit is less subject to influences of the same substrate, and this leads to that we can expect that there can be obtained a low-cost, high-performance high frequency device. Also in recent years, in silicon CMOS circuits for use in high frequencies, their usable upper-limit frequency has expanded to the GHz band, thus making it expected that higher-function, smaller-size high frequency modules are implemented by forming silicon CMOS active circuits and RF-MEMS passive circuits into monolithic circuits, respectively.
As a typical structure for reducing the dielectric loss of the substrate by using the RF MEMS technique, up to now, there has been disclosed such a structure that an interconnecting conductor is formed on a dielectric membrane support film, for example, in FIG. 1 of a prior art document of Stephen V. Robertson et al., “A 10-60-GHz Micromachined Directional Coupler,” IEEE Transactions on Microwave Theory & Techniques, Vol. 46, No. 11, p. 1845-1849, November 1998. In the shielded membrane microstrip line as disclosed in the above-mentioned prior art document (hereinafter, referred to as a second prior art), on a first semiconductor substrate having a grounding conductor film on its top surface, there is stacked a second semiconductor substrate, where a dielectric membrane support film having a strip conductor is formed on the top surface of the second semiconductor substrate, and an air space is formed on the bottom surface. Moreover, a further semiconductor substrate having a recessed portion in its bottom surface is stacked on the second semiconductor substrate. Then a microstrip line is made up.
In the membrane microstrip line according to the second prior art as constituted as shown above, when a high frequency signal is transmitted on the membrane microstrip line, an electromagnetic field of the high frequency signal is distributed in the dielectric membrane support film and an air layer of the air space which are located between the strip conductor and the grounding conductor film. In this case, since almost no electromagnetic field is generated in these semiconductor substrates, there can be obtained such an advantageous effect that the transmission loss can be reduced.
However, in the microstrip type millimeter waveguide according to the first prior art and the membrane microstrip line according to the second prior art, because of use of two or more semiconductor substrates, each of them has a complex structure and needs a complex manufacturing process, and this leads to such a problem that the manufacturing cost is increased. Furthermore, in these prior arts, there has been another problem that the transmission loss is still relatively high.